Common gate with resistive feed-through low noise amplifier

ABSTRACT

A radio-frequency amplifier is provided. The radio-frequency amplifier includes a transistor having an input terminal, an output terminal, a control terminal, and a transconductance g m . A series-connected feed-through resistance R f  and feed-through capacitance C f  is connected in parallel with the input terminal and the output terminal of the transistor. A load resistance R L  is connected to the output terminal. The control terminal of the transistor is biased at a fixed voltage. Part of the transistor noise follows the looped path through the feed-through resistor instead of passing on to the load, which reduces the noise figure of the amplifier. The value of g m , R f  and R L  are chosen in a way to keep the input impedance of the amplifier matched to a well-defined signal source impedance.

PRIORITY DATA

This application claims priority to U.S. provisional application No. 60/435,504, filed Dec. 20, 2002, entitled “Common Gate with Resistive Feedthrough Low Noise Amplifier,” which is hereby incorporated by reference for all purposes.

FIELD OF THE INVENTION

The present invention pertains to the field of amplifiers, and in particular to a common gate low noise amplifier with resistive feed-through for improved noise performance.

BACKGROUND OF THE INVENTION

The input stage of the low noise amplifier sets the limits on the sensitivity of the receiver. Therefore, low-noise is one of its most important design goals. Unfortunately, the lower intrinsic gain of transistors at higher frequencies makes it more difficult to achieve a low noise figure at very high frequencies. In such applications, additional noise sources such as gate-induced noise become more prominent with increasing frequency. The low noise amplifier also needs to achieve a sufficient gain to suppress the noise of the following stages and good linearity to handle out-of-band interference while providing a well-defined real impedance, which is normally 50-Ω.

In order to reduce the effect of noise at high frequency, a common-source stage with inductive degeneration has been used in CMOS low noise amplifier (“LNA”) implementations. It can be shown that for an input-matched common-source LNA, the minimum achievable noise factor, F_(min), and the effective transconductance, G_(m), are linearly related to the working frequency, Ω₀, and its inverse, 1/Ω₀, respectively. Although this common-source topology is well suited for applications at operating frequencies in the low GHz range, its performance degrades substantially at higher frequencies when Ω₀ becomes comparable to Ω_(T).

In contrast, in a common-gate (CG) LNA, the gate-source and gate-drain parasitic capacitances of the transistor are absorbed into the LC tank and resonated out at the operating frequency. Therefore, to the first order, the noise and gain performance of the common-gate stage is independent of the operating frequency, which is a desirable feature for high frequency design. However, due to the constraints of input matching, it can be shown that the noise factor of a common-gate LNA has a lower bound of 1+γ for perfect input match, where γ is the channel thermal noise coefficient. This represents a practical limit for noise reduction that restricts high-frequency applications.

SUMMARY OF THE INVENTION

In accordance with the present invention, a common-gate low noise amplifier with resistive feed-through is provided that overcomes known problems with high frequency amplifiers.

In particular, a common-gate low noise amplifier with resistive feed-through is provided that reduces noise effects while maintaining a well-matched input, and that is suitable for applications at either low frequencies or high frequencies.

In accordance with an exemplary embodiment of the present invention, a radio-frequency amplifier is provided. The radio-frequency amplifier includes a transistor having an input terminal, an output terminal, a control terminal, and a transconductance g_(m). A series-connected feed-through resistance R_(f) and feed-through capacitance C_(f) is connected in parallel with the input terminal and the output terminal of the transistor. A load resistance R_(L) is connected to the output terminal. The control terminal of the transistor is biased at a fixed voltage, such as ac ground. Part of the transistor noise follows the looped path through the feed-through resistor instead of passing on to the load, which reduces the noise figure of the amplifier. In addition, the values of g_(m), R_(f) and R_(L) keep the input impedance of a amplifier matched to a well-defined signal source impedance.

The present invention provides many important technical advantages. One important technical advantage of the present invention is a high-frequency amplifier with improved noise performance that allows radio-frequency signals to be amplified above 20 GHz without the introduction of significant amounts of noise.

Those skilled in the art will further appreciate the advantages and superior features of the invention together with other important aspects thereof on reading the detailed description that follows in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a common gate LNA with resistive feed-through in accordance with an exemplary embodiment of the present invention;

FIG. 2 is a diagram of a common gate LNA with resistive feed-through and a tank circuit in accordance with an exemplary embodiment of the present invention;

FIG. 3 is a gain analysis circuit in accordance with an exemplary embodiment of the present invention;

FIG. 4 is a small-signal equivalent circuit of a common gate resistive feed-through LNA in accordance with an exemplary embodiment of the present invention;

FIG. 5 is a substrate network model for a MOS transistor in accordance with an exemplary embodiment of the present invention;

FIG. 6 is a 24-GHz CMOS LNA in accordance with an exemplary embodiment of the present invention;

FIG. 7 is a diagram of a mixer in accordance with an exemplary embodiment of the present invention;

FIG. 8 is a table presenting measurements from experimental embodiments of an LNA and a mixer;

FIG. 9 shows measured input and output reflection coefficients, S₁₁ and S₂₂, for the experimental embodiments of an LNA and a mixer;

FIG. 10 shows the measured power gain and extracted voltage gain for the experimental embodiments of an LNA and a mixer with a 16.9-GHz local oscillator frequency;

FIG. 11 shows the measured large-signal nonlinearity for the experimental embodiments of an LNA and a mixer; and

FIG. 12 shows the measured noise figure for the experimental embodiments of an LNA and a mixer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the description that follows, like parts are marked throughout the specification and drawings with the same reference numerals, respectively. The drawing figures might not be to scale, and certain components can be shown in generalized or schematic form and identified by commercial designations in the interest of clarity and conciseness.

FIG. 1 is a diagram of a common gate LNA 100 with resistive feed-through in accordance with an exemplary embodiment of the present invention. Common gate LNA 100 can be implemented in CMOS or in other suitable manners.

Common gate LNA 100 includes transistor 102, which can be a CMOS transistor or other suitable devices. Feed-through resistor 104 is provided in parallel with amplifier 102, and a feed-through capacitance 106 is used to isolate dc level. The gate of transistor 102 is biased at a fixed voltage, and an inductor 108 provides a dc current path from the input terminal to ground.

Feed-through resistor 104 forms a closed loop with the transistor 102 channel thermal noise (or shot noise) source. As such, part of the noise signal follows a looped path through feed-through resistor 104 instead of passing on to the load, which reduces the noise figure of the LNA.

In operation, common gate LNA 100 with feed-through path formed by resistor 104 and capacitance 106 provides for improved performance degradation at higher frequencies. The output noise power due to the transistor channel noise can be lowered towards 0 by reducing resistor 104. The dc current needs to be increased to maintain the gain and input matching. By this means, the topology shown provides a direct way to trade between power dissipation and the noise figure, such that common gate LNA 100 with a feed-through path provides improved performance at very high frequencies where the noise figure takes precedence over power dissipation.

FIG. 2 is a diagram of a common gate LNA 200 with resistive feed-through and a tank circuit in accordance with an exemplary embodiment of the present invention. As shown, R_(s) is the signal source impedance, C_(p) is a large capacitor for isolating dc level, and R_(L) is the resistive load at the drain of M₁. Inductors L_(L) and L_(s) resonate at the operating frequency with a capacitive load at the drain and source of M₁, respectively.

FIG. 3 is a gain analysis circuit 300 in accordance with an exemplary embodiment of the present invention. The effective transconductance g_(m) equals 1/(2 * R_(s)). It should also be noted that g_(m) is independent of R_(f) for a matched input.

FIG. 4 is a small-signal equivalent circuit 400 of a common gate resistive feed-through LNA in accordance with an exemplary embodiment of the present invention. In addition to the major noise sources (represented as i_(n) current sources), g_(m) is the transistor transconductance, g_(mb) is the backgate transconductance, g_(g) is the real part of the gate admittance, i_(n,d) ² is the transistor channel thermal noise source, and i_(n,g) ² is the induced gate noise source. At low frequency, where i_(n,g) ² and g_(g) can be neglected, the feedthrough resistor R_(f) (which is formed by an external parallel resistance R_(p) in parallel with the drain-to-source resistance r_(ds)) creates a positive feedback loop around the amplifier to enhance the input impedance. At resonance frequency, the input impedance seen looking into the source of M₁ can be expressed as: $\begin{matrix} {Z_{i\quad n} = \frac{R_{f} + R_{L}}{1 + {g_{m}{R_{f}\left( {1 + \chi} \right)}}}} & (1) \end{matrix}$ where g_(m) is the transistor transconductance and χ is the ratio of the transistor backgate transconductance g_(mb) to g_(m). If the input is matched, the effective transconductance of the common gate resistive feed-through stage is given by: $\begin{matrix} {G_{m,{CGRF}} = \frac{1}{2R_{s}}} & (2) \end{matrix}$ which indicates that to the first order at the input matching condition, the gain of the common gate resistive feed-through stage is independent of R_(f) and g_(m).

Assuming a matched input and R_(s)<<R_(L), the output noise power generated by the thermal noise of R_(p) and R_(L) is negligible compared to that generated by the transistor channel thermal noise, in which case the noise factor can be approximately expressed as: $\begin{matrix} {F_{CGRF} \approx {1 + {\frac{\gamma}{\alpha}\left( \frac{1}{1 + \chi} \right)^{2}\frac{1}{g_{m}R_{s}}}}} & (3) \end{matrix}$ where α is the ratio of g_(m) to the channel conductance at zero drain-to-source voltage, g_(d0).

Based on the simplifying assumptions that gate noise and g_(g) may be ignored, the noise of the common gate resistive feed-through amplifier approaches 0 dB by increasing g_(m), providing a direct way to trade between power and noise while keeping the input matched. However, at high frequencies, additional effects need to be considered, such as that the coupling between channel and gate is due to a distributed RC network, reflected in the real part of the gate admittance, g_(g). In the pinch-off region, g_(g) is related to operation frequency Ω₀, gate-source capacitance, C_(gs), and g_(d0) through: $\begin{matrix} {g_{g} = \frac{C_{gs}^{2}\omega_{0}^{2}}{5g_{d\quad 0}}} & (4) \end{matrix}$ This conductance has a thermal noise, i_(n,g) ² , associated with it, which is called induced gate noise. The power spectral density of i_(n,g) ² is given by: $\begin{matrix} {\frac{\overset{\_}{i_{n,g}^{2}}}{\Delta\quad f} = {4{kT}\quad\delta\quad g_{g}}} & (5) \end{matrix}$ where δis the gate noise coefficient, and i_(n,d) ² and i_(n,g) ² are partially correlated with a complex correlation coefficient c given by: $\begin{matrix} {c = \frac{\overset{\_}{i_{n,g}i_{n,d}^{*}}}{\sqrt{\overset{\_}{i_{n,g}^{2}i_{n,d}^{2}}}}} & (6) \end{matrix}$ Taking g_(g) into account, the input impedance of the common gate resistive feed-through stage is revised as: $\begin{matrix} {Z_{i\quad n} = \left( {\frac{1 + {g_{m}{R_{f}\left( {1 + \chi} \right)}}}{R_{f} + R_{L}} + {{\eta\left( \omega_{0} \right)}g_{m}}} \right)^{- 1}} & (7) \end{matrix}$ where η(Ω₀) is defined as the ratio between g_(g) and g_(m): $\begin{matrix} {{\eta\left( \omega_{0} \right)} \equiv \frac{g_{g}}{g_{m}} \approx {\frac{\alpha}{5}\left( \frac{\omega_{0}}{\omega_{T}} \right)^{2}}} & (8) \end{matrix}$ If input is perfectly matched to R_(s), the effective transconductance of the common gate resistive feed-through stage is given by: $\begin{matrix} {G_{m,{CGRF}} = {\frac{1}{2R_{s}}\left\lbrack {1 - {g_{m}R_{s}{\eta\left( \omega_{0} \right)}}} \right\rbrack}} & (9) \end{matrix}$ which indicates that a large g_(m) and high frequency can degrade the gain. This is because the increase of g_(m) results in a larger g_(g) causing more signal loss through the gate.

If a matched input is provided and R_(s)<<R_(L), the following expression defines the value of F: $\begin{matrix} {F_{CRGF} \approx {1 + {\frac{\gamma}{\alpha}\left( \frac{1}{1 + \chi} \right)^{2}\left( {\frac{1}{g_{m}R_{s}} + {{\eta^{2}\left( \omega_{0} \right)}g_{m}R_{s}} + {2{\eta\left( \omega_{0} \right)}}} \right)} + {{{\delta\eta}\left( \omega_{0} \right)}g_{m}R_{S}}}} & (10) \end{matrix}$ where the second term represents the contribution of channel thermal noise and the third term represents the contribution of induced gate noise. At low frequencies, η(Ω₀)→0, and equation (10) reduces to equation (3).

An optimum g_(m) exists for minimum noise figure, i.e., $\begin{matrix} {g_{m,{CGRF},{opt}} = {\frac{1}{R_{s}}\left( {{\frac{\delta\quad\alpha}{\gamma}\left( {1 + \chi} \right)^{2}{\eta\left( \omega_{0} \right)}} + {\eta^{2}\left( \omega_{0} \right)}} \right)^{\frac{1}{2}}}} & (11) \end{matrix}$ And the corresponding minimum F is approximately given by: $\begin{matrix} {F_{{CGRF},\min} \approx {1 + {\gamma\left( {{\frac{1}{1 + \chi}\sqrt{\frac{4\delta}{5\gamma\quad\alpha}}\left( \frac{\omega_{0}}{\omega_{T}} \right)} + {\frac{1}{\alpha\left( {1 + \chi} \right)}\frac{2}{5}\left( \frac{\omega_{0}}{\omega_{T}} \right)^{2}}} \right)}}} & (12) \end{matrix}$

Since R^(f) results in positive feedback in the common gate resistive feed-through stage, stability needs to be addressed. Considering the input transistor with feed-through resistor as a two-port network, Z_(s) and Z_(L) are the load impedance at the two ports, source and drain, respectively. It is a sufficient condition to prevent oscillation that the real part of both impedances seen looking into the ports, Z_(in) and Z_(out), are positive. Re[Z_(in)] and Re[Z_(out)] can be expressed as $\begin{matrix} {{{Re}\left\lbrack Z_{in} \right\rbrack} = \frac{R_{f} + {{Re}\left\lbrack Z_{L} \right\rbrack}}{1 + {g_{m}{R_{f}\left( {1 + \chi} \right)}}}} & (13) \\ {{{Re}\left\lbrack Z_{out} \right\rbrack} = {R_{f} + {\left\lbrack {{g_{m}{R_{f}\left( {1 + \chi} \right)}} + 1} \right\rbrack{{Re}\left\lbrack Z_{S} \right\rbrack}}}} & (14) \end{matrix}$ where R_(f)=R_(p)∥r_(ds), and equations (13) and (14) indicate that as long as Re[Z_(L)] and Re[Z_(s)] are positive, stability of the common gate resistive feed-through stage is provided.

FIG. 5 is a substrate network model 500 for a MOS transistor in accordance with an exemplary embodiment of the present invention. Capacitive coupling between the drain and source through this network adversely affects stability and the noise figure. A shunt inductor L_(p) in series with a large bypass capacitor C_(p) resonates the equivalent capacitance between drain and source so that the substrate effects are reduced. The series resistance of L_(p) can be converted to an equivalent parallel resistance, which affects the performance of the LNA as a feed-through resistor. In this case, the feed-through resistance can be expressed as R _(f) =Qω ₀ L _(p) ∥r _(ds)  (15) where R_(LP) is the series resistance of L_(p).

FIG. 6 is a 24-GHz CMOS LNA 600 in accordance with an exemplary embodiment of the present invention. An experimental embodiment of LNA 600 was also constructed, and test results from tests performed on that experimental embodiment are discussed herein in greater detail.

LNA 600 includes three stages. The first stage employs a common-gate with resistive feed-through topology, where shunt inductor L₂ resonates the capacitive coupling between the drain and the source of M₁, while its parasitic resistance R_(L2) introduces a feed-through resistance described by equation (15). A large capacitor C₂ isolates the dc level of the source and the drain. The second and third stages are both common-source with inductive degeneration amplifiers, which are used to enhance the overall gain.

The peak f_(T) of the 0.18-μm CMOS device used at 1.5 V bias is about 60 GHz. To achieve the minimum noise figure at 24 GHz, the optimum g_(m1) is estimated to be about 80 mS in accordance with equation (11). To reduce the power consumption, g_(m1) is set to 40 mS. (V_(gs)−V_(t)) is also lowered by a factor of two from its value of peak f_(T), which is a power efficient way for reducing current consumption by more than 50%, while reducing f_(T) by about 10% only. Finally M₁ is biased at 8 mA with 54 GHz f_(T). The second and third stages consume 4 mA each.

Since the feed-through resistor is replaced by an inductor in the first stage, the stability of the amplifier needs to be reexamined. Computer circuit simulations show that the first stage is stable up to 43 GHz. Above 43 GHz, the stability factor of the stage, K_(f), is less than one. However, the input impedance of the second stage is located in the stable region with sufficient margin for stable operation in the frequency range of interest.

FIG. 7 is a diagram of a mixer 700 in accordance with an exemplary embodiment of the present invention. A conventional single-balanced Gilbert cell is used, and the RF input is applied at the gate of M₄ which is used as a transconductance amplifier. The linearity of this transconductance amplifier is improved by using source degeneration inductor L₈, which also adjusts the input impedance seen looking into the gate of M₄ in order to improve the power matching at the LNA-mixer interface. M₄ is biased at 4 mA dc current.

The chopping function of mixer 700 is accomplished by the M₂˜M₃ mixing cell and a 1.6 V peak-to-peak differential LO signal is applied. Cascode amplifiers following the differential mixing cell are used to drive the 50-Ω loads. The output-match is accomplished by the LC impedance transforming network.

FIG. 8 is a table 800 presenting measurements from experimental embodiments of LNA 600 and mixer 700. The circuits were designed and fabricated using 0.18-μm CMOS transistors. The process used to fabricate the experimental embodiments used six metal layers with two top layers of 1-μm thick copper. The inductors L₄ and L₆ of LNA 600 and L₈ of mixer 700 were implemented as slab inductors, and the remaining inductors were implemented as spirals. Shielded pads were employed at both transmission frequency and intermediate frequency ports. Grounded metal underneath the pads prevented loss of the signal power and noise generation associated with substrate resistance.

Ground rings were placed around each transistor at a minimum distance to reduce the substrate loss. Separated V_(dd) pads were assigned to LNA 600, mixer 700, and bias circuits. Large on-chip bypass capacitors were placed between each V_(dd) and ground. The size of the chip was 0.8×0.9 mm including a large area occupied by the wide ground rings and pads. The size of the core cell was only 0.4×0.5 mm.

The image rejection of the front-end was −31 dB. This performance was achieved via the large intermediate frequency and the multi-stage nature of LNA 600. The overall current consumption of the front-end was 43 mA, out of which the output buffers consumed 23 mA. The experimental embodiments of LNA 600 and mixer 700 drew 16 mA and 4 mA, respectively, from a 1.5-V supply voltage.

FIG. 9 shows the measured input and output reflection coefficients, S₁₁ and S₂₂, for the experimental embodiments of LNA 600 and mixer 700. As shown by these measurements, the radio frequency input and the intermediate frequency output are well matched at the respective frequencies.

FIG. 10 shows the measured power gain and extracted voltage gain for the experimental embodiments of LNA 600 and mixer 700 with a 16.9-GHz local oscillator frequency. The measurement shows that a 27.5 dB maximum power gain appears for a transmission frequency of 21.8 GHz and an intermediate frequency of 4.9 GHz. The frequency offset from the 24 GHz is likely due to inaccurate modeling of the MOS transistor and the planar inductor at high frequencies. The experimental embodiment of LNA 600 achieved a 15-dB power gain. The experimental embodiment of mixer 700 further enhanced the signal power by 13 dB. Because of the imperfect conjugate-matching at the LNA-mixer interface, the overall power gain of the front-end was slightly lower than the sum of the individual power gain of the two blocks.

FIG. 11 shows the measured large-signal nonlinearity for the experimental embodiments of LNA 600 and mixer 700. The input referred 1 dB compression points appears at −23 dBm.

FIG. 12 shows the measured noise figure for the experimental embodiments of LNA 600 and mixer 700. A minimum noise figure of 7.7 dB was achieved for the combined LNA 600 and mixer 700 at 22.08 GHz. The individual noise figures of LNA 600 and mixer 700 were 6 dB and 17.5 dB, respectively. The noise figure of the first common gate resistive feed-through stage was extracted to be 4.8 dB. Applying equation (3) predicts the i_(n,d) ² only noise figure of the first stage to be 3.3 dB. Applying equation (10) adjusts the prediction of the noise figure to be 4.1 dB by taking g_(g) and i_(n,g) ² into account, and the remaining amount of 0.7 dB is due to the thermal noise of the parasitic resistance and substrate noise.

Although exemplary embodiments of a system and method of the present invention have been described in detail herein, those skilled in the art will also recognize that various substitutions and modifications can be made to the systems and methods without departing from the scope and spirit of the appended claims. 

1-15. (canceled)
 16. A radio-frequency amplifier comprising: a transistor having an input terminal, an output terminal and a control terminal; a series-connected feed-through resistance and feed-through capacitance connected in parallel with the input terminal and the output terminal of the transistor; a load impedance connected to the output terminal of the transistor; and wherein the control terminal of the transistor is coupled to common voltage.
 17. The radio-frequency amplifier of claim 16 further comprising a tank circuit connected between a voltage source V_(dd) and the output terminal of the transistor.
 18. The radio-frequency amplifier of claim 16 driven by a signal source in series with an with impedance R_(s) coupled between the input terminal and common voltage, wherein a transconductance g_(m) of the transistor is larger than 1/R_(s).
 19. The radio-frequency amplifier of claim 16 wherein the feed-through resistance is formed by a resistor R_(p) in parallel with the transistor drain-source small-signal resistance r_(ds).
 20. The radio-frequency amplifier of claim 18 wherein the feed-through resistance further comprises a resonant capacitive-inductive circuit having an effective resistance at an operating frequency.
 21. The radio-frequency amplifier of claim 16 wherein the transistor further comprises transconductance means for reducing noise.
 22. The radio-frequency amplifier of claim 16 wherein the load impedance comprises a parallel combination of a resistance, a capacitance, and an inductance.
 23. The radio-frequency amplifier of claim 16 wherein the radio frequency amplifier is coupled to a radio-frequency receiver.
 24. A high-frequency CMOS low noise amplifier comprising: a first stage including a common-gate amplifier having a source and a drain with a feed-through resistance coupled between the source and the drain; a second stage coupled to the first stage including a common-source amplifier with inductive degeneration; and a third stage coupled to the second stage including a common-source amplifier with inductive degeneration.
 25. The high-frequency CMOS low noise amplifier of claim 24 wherein the feed-through resistance is formed by a resistance in parallel with a transistor drain-source resistance.
 26. The high-frequency CMOS low noise amplifier of claim 24 wherein the feed-through resistance further comprises a resonant capacitive-inductive circuit having an effective resistance at an operating frequency.
 27. The high-frequency CMOS low noise amplifier of claim 24 driven by a signal source with output impedance R_(s), wherein a transconductance g_(m) of the input transistor is larger than 1/R_(s), and a series-connected resistor R_(f) and capacitor C_(f) is connected between the input terminal and the output terminal of the input transistor, so that the real part of an input impedance of the high-frequency CMOS low noise amplifier is increased.
 28. The radio-frequency amplifier of claim 16 wherein the common-gate amplifier comprises transconductance means for reducing noise.
 29. The high frequency CMOS low noise amplifier of claim 24 wherein the high-frequency CMOS low noise amplifier is coupled to a radio-frequency receiver.
 30. A radio-frequency amplifier comprising: an amplifier having an input, an output, and a gate connected to voltage common; and a resistive feed-through circuit having a resistance coupled in parallel with the input and the output of the common gate amplifier, wherein the resistive feed-through circuit reduces output noise power.
 31. The radio-frequency amplifier of claim 30 wherein the resistive feed-through circuit further comprises a resonant capacitive-inductive circuit having an effective resistance at an operating frequency.
 32. The radio-frequency amplifier of claim 30 wherein the resistive feed-through circuit comprises a feed-through resistance in series with a feed-through capacitance.
 33. The radio-frequency amplifier of claim 30 further comprising: a signal source impedance; and transconductance means for reducing noise.
 34. The radio-frequency amplifier of claim 30 wherein the radio frequency amplifier is coupled to a radio-frequency receiver. 